1. Field of the Invention
The present invention relates to: a toner used as a developer when an electrostatic image formed by electrophotography, electrostatic recording and so on is developed; a two-component developer including the toner; and an image forming apparatus using the toner.
2. Description of the Related Art
After a charging step which uniformly charges an image-forming region on a surface of an image bearing member, an exposing step which writes on the image bearing member, a developing step which forms an image on the image bearing member by a frictionally charged toner and a transfer step which transfers the image on the image bearing member directly on a printing sheet or indirectly via an intermediate transfer member, an image forming apparatus fixes the image on the printing sheet. Also, a transfer residual toner not transferred on the image bearing member, is scraped from the image bearing member by a cleaning step, and a next image forming process is carried out.
As the developer to be used, there are a two-component developer composed of a toner and a carrier and a one-component developer composed only of a magnetic or a non-magnetic toner. In general, these toners are manufactured by a melt-kneading pulverization method, where a resin, a pigment, a charge controlling agent and a releasing agent are melt-kneaded, followed by cooling, pulverization and classification, but a particle diameter and a shape of the toner are not uniform, and it is difficult to control them.
Under such circumstances, there is an attempt to intentionally control a particle diameter of toner particles in recent years, trying to solve the aforementioned problems, and toner polymerization methods such as emulsion-polymerization method and dissolution-suspension method became popular as aqueous granulation.
In recent years, due to increased demand for higher quality, especially to achieve a high-definition image in color image formation, there is a growing demand for reduction and homogenization of the toner particle diameter. When an image is formed using a toner having a wide particle-diameter distribution, problems of contamination of a developing roller, a charging roller, a charging blade, a photoconductor, a carrier and so on by fine-powder toner and toner scattering become severe, and it is difficult to fulfill high quality high reliability at the same time. On the other hand, when the particle diameter is uniform and the particle diameter distribution is sharp, developing behavior of individual toner particles becomes uniform, and fine-dot reproducibility significantly improves.
In general, as a fixing system in the electrophotography, a heat-roller heating system that a heat roller is directly pressed on a toner image on a recording medium for fixing is widely used in terms of energy efficiency. The heat-roller heating system requires a large amount of electric power for fixing. Thus, in view of saving energy, reduction of energy consumption of the heat roller has been studied. For example, a system that an output of a heater for a heat roller is reduced when an image is not being output and that a temperature of the heat roller is increased by increasing the output of the heater when an image is being output is generally used.
However, in this case, in order to increase the temperature of the heat roller from a sleep mode to a temperature required for fixing, a standby time of around several tens of seconds is required, and this standby time is stressful for users. Also, when an image is not output, it is desired to suppress energy consumption by completely turning off the heater. To respond to these requests, it is necessary to reduce a fixing temperature of a toner itself and to reduce the fixing temperature of the toner available for fixing.
With the development of electrophotographic technologies, a toner used in the developer is required to have superior low-temperature fixing property, storage stability (blocking resistance) and stress resistance, there have been various attempts to use a polyester resin having high compatibility with a recording medium and so on and superior low-temperature fixing property compared to a styrene resin which has been generally and conventionally used as a binder resin for a toner.
However, a toner designed with an emphasis on low-temperature fixing property has a trade-off relationship with storage stability and stress resistance by softening the resin, and it is required to achieve the both.
In order to solve this problem, a toner having a capsule structure composed of core particles including a resin having a low glass transition temperature, and an outer shell which is formed to coat a surface of the core particles and includes a resin having a high glass transition temperature has been proposed.
For example, a capsule toner is proposed, wherein a mixed solution of a core-material constitutional material including a monomer (polymerizable monomer) as a raw material of a thermoplastic resin and an outer-shell constitutional material including non-crystalline polyester is dispersed in a dispersion medium, and by an in-situ polymerization method, in parallel with a formation of core particles by polymerization, an outer shell is formed by distributing unevenly the outer shell constitutional material on a surface of liquid droplets (see Japanese Patent (JP-B) No. 3030741).
Also, a toner is proposed, wherein an aqueous dispersion liquid of resin particles obtained by emulsion polymerization or soap-free emulsion polymerization is added to polymer particles (core particles) obtained by suspension polymerization so that 95% or more of a surface of the polymer particles is covered by the fine particles, and then it is heated to a temperature of a glass transition temperature of the polymer particles or higher so that the surface has substantially no asperity (see Japanese Patent Application Laid-Open (JP-A) No. 2000-112174).
Also, a toner is proposed, wherein resin particles having at least two different glass transition temperatures and a toner core material (core particles) are mixed, and a coating resin is disposed on the toner core material by fixing or fusing the resin particles while increasing the temperature (see JP-A No. 2001-201891).
Further, a toner obtained by a process including a first step and a second step is proposed, wherein a surface of a core toner (core particles) composed of a binder resin having an average particle diameter of 2 μm to 20 μm and a glass transition temperature of 30° C. to 55° C. is coated with resin particles encapsulating a wax followed by fixing or fusing in the first step and is coated with resin particles which does not include a wax followed by fixing or fusing in the second step (see JP-A No. 2001-235894).
However, in the patent literatures described above, it is possible to improve the problems of storage stability, blocking resistance and aggregation property under a high temperature, but measures for stresses in the developing step have not been taken, degradation of the toner due to developing stresses, and degradation of transfer properly, developing property and cleanability attributed thereto are concerned.
In order to improve transfer property, developing property and cleanability, it is disclosed to combine inorganic fine particles having a medium particle diameter with an average particle diameter of 20 μm to 40 μm as an external additive (see JP-A No. 03-100661).
Also, it is disclosed to use inorganic fine particles having a large particle diameter in order to suppress embedding of an external additive due to stresses in a developing machine (see JP-B No. 3328013, JP-A No. 09-319134, JP-B No. 3056122).
With these, favorable cleanability, transfer property and developing property may be obtained initially, but adhesive strength of the inorganic fine particles vary from particles to particles, and the inorganic fine particles liberate over time, causing contamination in the developing machine or around a photoconductor or resulting in insufficient transfer property and cleanability.
On the other hand, as a means to reduce non-electrostatic adhesion between toner particles and an electrophotographic photoconductor or between toner particles and an intermediate transfer member, a method to adjust a type or an amount of an external additive (especially, adding an external additive having a large particle diameter) is proposed (see JP-A No. 08-176310). With this method, it is possible for the toner particles to improve transfer efficiency with an effect of reduced non-electrostatic adhesion, and at the same time, it is possible to obtain effects such as improved development stability and cleaning.
Further, with reduction of particle diameter of a toner and control of a shape of toner particles in recent years, an added amount of an external additive increases, there are problems of filming, carrier contamination and so on. Also, with toner particles having a shape of a complex structure, it is initially possible to produce a high-quality image, but it becomes difficult to maintain the high-quality image over time due to the external additive embedded or the external additive rolling into concave portions. Especially, in a case where a fine irregular structure on a surface of the toner becomes large, loss of functions increases due to the embedding or rolling of the external additive. Also, when an external additive having a large particle diameter is added, supply property of the toner is affected due to small improvement effect of toner fluidity.
The toner particles described above can initially improve transfer efficiency of an image forming apparatus. However, the toner receives mechanical stresses such as stirring over a long period of time in a developing apparatus of the image forming apparatus, causing the external additive embedded in the toner base or rolling into concave portions of a surface of the toner base. As a result, an effect of reduced adhesion by the external additive is not exhibited, and transfer efficiency of the image forming apparatus decreases. Especially, in a case of a high-speed machine, this mechanical stresses is large due to vigorous stirring in a developing apparatus, and it is likely that embedding of the external additive in the toner base is accelerated. Thus, it is expected that transfer efficiency is reduced at a relatively early stage. In recent years, it is disclosed to suppress embedding by using an external additive having a relatively large particle diameter, but there are problems that an effect of imparting toner fluidity is low as described above and that the free external additive causes filming.
Thus, in order to maintain high transfer efficiency in a high-speed machine in a stable manner over a long period of time, it is necessary to control surface property (mechanical strength) of a toner so that an external additive exists on the surface without being embedded into a toner base despite receiving mechanical stresses. Further, when the surface property (mechanical strength) of the toner is excessively strengthened (hardened), attention should be paid to side effects of degraded fixability, e.g. inhibition of toner melting during fixing or insufficient bleeding of a releasing agent on a fixing roller during fixing in a case of a toner including a releasing agent such as wax and so on. Further, it is possible to maintain high transfer rate by a simple spheronization process of a toner, but it causes a side effect of reduced cleanability of the toner.
Also, for the purpose of improving low-temperature fixing property, a method of introducing crystalline polyester to a polymerization method is disclosed. As a method for preparing a dispersion liquid of crystalline polyester, a method for preparing a dispersion liquid using a solvent for phase separation is disclosed (see JP-B No. 3328013). By using crystalline polyester, it is possible to achieve low-temperature fixing property. However, this is insufficient because an external additive is likely to be embedded with the toner including crystalline polyester, resulting in decrease in transfer efficiency.
Also, use of a non-spherical external additive for improving image density stability is disclosed (see JP-B No. 3684074, JP-A No. 2010-224502). It is possible to achieve improved transfer efficiency by the non-spherical external additive. However, an amount of adhesion of a toner decreases when an aggregate of non-spherical particles are present. Aggressiveness toward a photoconductor increases due to the aggregate of free non-spherical particles, and it causes scratches on the photoconductor. However, such a problem of is not mentioned.
In other words, a spherical toner having a small particle diameter has been developed by aqueous granulation in recent years, but there remains a challenge to cleanability. Surface irregularities increases with toner base particles having a high BET specific surface area, and it is advantageous in cleanability. Also, it reduces an effective coverage of an external additive, and it is effective for low-temperature fixing property. However, for a toner having a high specific surface area relative to toner base, there are many cases where an external additive cannot sufficiently exhibit its effect under stresses due to its irregularities because of the external additive rolling into concave portions or the external additive embedded in convex portions.